U.S. patent application number 14/076571 was filed with the patent office on 2015-05-14 for polarization splitter/combiner based on a one-dimensional grating coupler.
This patent application is currently assigned to Futurewei Technologies, Inc.. The applicant listed for this patent is Futurewei Technologies, Inc.. Invention is credited to Xiao Shen, Qianfan Xu.
Application Number | 20150131942 14/076571 |
Document ID | / |
Family ID | 53040921 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150131942 |
Kind Code |
A1 |
Xu; Qianfan ; et
al. |
May 14, 2015 |
Polarization Splitter/Combiner Based On A One-Dimensional Grating
Coupler
Abstract
A grating coupler comprising a semiconductor substrate, a
one-dimensional (1D) grating element coupled to the semiconductor
substrate, wherein the 1D grating element is adapted to
simultaneously couple a first polarization component of an incident
optical beam with a transverse electric (TE) waveguide mode in a
first propagation direction and a second polarization component of
the incident optical beam with a transverse magnetic (TM) waveguide
mode in a second propagation direction, and wherein the first
propagation direction is opposite of the second propagation
direction.
Inventors: |
Xu; Qianfan; (San Jose,
CA) ; Shen; Xiao; (San Bruno, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Futurewei Technologies, Inc. |
Plano |
TX |
US |
|
|
Assignee: |
Futurewei Technologies,
Inc.
Plano
TX
|
Family ID: |
53040921 |
Appl. No.: |
14/076571 |
Filed: |
November 11, 2013 |
Current U.S.
Class: |
385/37 |
Current CPC
Class: |
G02B 6/125 20130101;
G02B 6/34 20130101; G02B 6/305 20130101; G02B 6/2726 20130101; G02B
6/1226 20130101; G02B 6/124 20130101 |
Class at
Publication: |
385/37 |
International
Class: |
G02B 6/34 20060101
G02B006/34 |
Claims
1. A grating coupler comprising: a semiconductor substrate; a
one-dimensional (1D) grating element coupled to the semiconductor
substrate, wherein the 1D grating element is adapted to
simultaneously couple a first polarization component of an incident
optical beam with a transverse electric (TE) waveguide mode in a
first propagation direction and a second polarization component of
the incident optical beam with a transverse magnetic (TM) waveguide
mode in a second propagation direction, and wherein the first
propagation direction is opposite of the second propagation
direction.
2. The grating coupler of claim 1, wherein the TE waveguide mode
comprises a first diffraction angle, wherein the TM waveguide mode
comprises a second diffraction angle, and wherein a sum of the
first diffraction angle and the second diffraction angle equals
about 180 degrees.
3. The grating coupler of claim 1, further comprising an integrated
waveguide disposed between the 1D grating element and the
substrate, and wherein the integrated waveguide comprises a lower
cladding layer and a core layer coupled to the lower cladding
layer.
4. The grating coupler of claim 3, wherein the lower cladding layer
comprises silicon dioxide, and wherein the core layer comprises
silicon.
5. The grating coupler of claim 1, wherein the 1D grating element
comprises a grating period, wherein the grating period (A) is
determined by: .LAMBDA. = 2 .lamda. n eff TE + n eff TM ,
##EQU00004## wherein .lamda. is a center wavelength of the incident
optical beam, wherein .eta..sub.eff.sup.TE is an effective index of
the TE waveguide mode, and wherein .eta..sub.eff.sup.TM is an
effective index of the TM waveguide mode.
6. The grating coupler of claim 5, wherein the center wavelength is
in the range of about 1100 nanometers (nm) to 2500 nm.
7. The grating coupler of claim 6, wherein the grating coupler is
adapted to provide a 1 decibel (dB) bandwidth of greater than 62.5
nm.
8. The grating coupler of claim 5, wherein the 1D grating element
further comprises an occupation ratio and a spatial period, and
wherein the occupation ratio, the spatial period, or a combination
thereof may be varied.
9. The grating coupler of claim 8, wherein the grating coupler is
adapted to provide a 1 dB bandwidth of greater than 46 nm.
10. The grating coupler of claim 1, wherein the grating coupler is
adapted to combine a TE polarized signal from the first propagation
direction with a TM polarized signal from the second propagation
direction into a single optical beam.
11. An apparatus comprising: an optical element configured to
communicate with a grating coupler via an optical medium, wherein
the grating coupler comprises: a semiconductor substrate; a
one-dimensional (1D) grating element coupled to the semiconductor
substrate; and an integrated waveguide disposed between the 1D
grating element and the substrate, wherein the 1D grating element
is adapted to simultaneously couple a first polarization component
of an optical beam with a transverse electric (TE) waveguide mode
in a first propagation direction and a second polarization
component of the optical beam with a transverse magnetic (TM)
waveguide mode in a second propagation direction, and wherein the
first propagation direction is opposite of the second propagation
direction.
12. The apparatus of claim 11, wherein the optical element is an
optical receiver, wherein the optical receiver receives the optical
beam from the grating coupler, and wherein the optical beam is
formed by the grating coupler combining a TE polarized signal from
the first propagation direction with a TM polarized signal from the
second propagation direction.
13. The apparatus of claim 11, wherein the optical element is an
optical transmitter configured to transmit the optical beam to the
grating coupler.
14. The apparatus of claim 11, wherein the optical medium is an
optical fiber.
15. The apparatus of claim 11, wherein the optical medium is a
free-space optical system.
16. The apparatus of claim 11, wherein the 1D grating element
comprises a grating period, wherein the grating period (.LAMBDA.)
is determined by: .LAMBDA. = 2 .lamda. n eff TE + n eff TM ,
##EQU00005## wherein .lamda. is a center wavelength of the optical
beam, wherein .eta..sub.eff.sup.TE is an effective index of the TE
waveguide mode, and wherein .eta..sub.eff.sup.TM is an effective
index of the TM waveguide mode.
17. The apparatus of claim 11, wherein the TE waveguide mode
comprises a first diffraction angle, wherein the TM waveguide mode
comprises a second diffraction angle, and wherein a sum of the
first diffraction angle and the second diffraction angle equals
about 180 degrees.
18. A method comprising: coupling a first polarization component of
an incident optical beam with a transverse electric (TE) waveguide
mode in a first propagation direction with a one-dimensional (1D)
grating element; coupling a second polarization component of the
incident optical beam with a transverse magnetic (TM) waveguide
mode in a second propagation direction with the 1D grating element,
wherein the first propagation direction is opposite of the second
propagation direction, and wherein the first polarization component
is coupled simultaneously with the second polarization
component.
19. The method of claim 18, wherein the TE waveguide mode comprises
a first diffraction angle, wherein the TM waveguide mode comprises
a second diffraction angle, and wherein a sum of the first
diffraction angle and the second diffraction angle equals about 180
degrees.
20. The method of claim 19, wherein the 1D grating element
comprises a grating period, wherein the grating period (.LAMBDA.)
is determined by: .LAMBDA. = 2 .lamda. n eff TE + n eff TM ,
##EQU00006## wherein .lamda. is a center wavelength of the incident
optical beam, wherein .eta..sub.eff.sup.TE is an effective index of
the TE waveguide mode, and wherein .eta..sub.eff.sup.TM is an
effective index of the TM waveguide mode.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not applicable.
BACKGROUND
[0004] Grating couplers as key elements in silicon photonic systems
have been used to couple optical signals in and/or out of a planar
silicon waveguide fabricated on a chip from or to an out-of-plane
optical beam. The out-of-plane optical beam may then be coupled to
optical fiber communication systems either directly or through a
free-space optical system. One-dimensional (1D) grating couplers,
formed by silicon rails and trenches, have been used to couple
light of a single polarization. In a fiber optical system, the
polarization of light in an optical fiber is random; therefore,
light of a single-polarization may not be used on a receiver side
to couple light from the optical fiber onto a silicon waveguide.
Two-dimensional (2D) grating couplers, formed by a 2D post and/or
hole array have been used to simultaneously couple light at two
polarizations. The structure of the 2D grating coupler may separate
two polarizations from the optical signal and forward each into
separate waveguides, which may be referred to as polarization
splitting. Each of the polarizations in their respective waveguides
may then be separately processed before they are forwarded to a
single detector. However, 2D grating couplers may have both a lower
coupling efficiency of below -3 decibels (dBs) and a lower optical
bandwidth than their 1D grating coupler counterparts. Consequently,
2D grating couplers may limit the performance of optical fiber
communication systems.
SUMMARY
[0005] In one embodiment, the disclosure includes a grating coupler
comprising a semiconductor substrate, a 1D grating element coupled
to the semiconductor substrate, wherein the 1D grating element is
adapted to simultaneously couple a first polarization component of
an incident optical beam with a transverse electric (TE) waveguide
mode in a first propagation direction and a second polarization
component of the incident optical beam with a transverse magnetic
(TM) waveguide mode in a second propagation direction, and wherein
the first propagation direction is opposite of the second
propagation direction.
[0006] In another embodiment, the disclosure includes an apparatus
comprising an optical element configured to communicate with a
grating coupler via an optical medium, wherein the grating coupler
comprises a semiconductor substrate, a 1D grating element coupled
to the semiconductor substrate, and an integrated waveguide
disposed between the 1D grating element and the substrate, wherein
the 1D grating element is adapted to simultaneously couple a first
polarization component of an optical beam with a TE waveguide mode
in a first propagation direction and a second polarization
component of the optical beam with a TM waveguide mode in a second
propagation direction, and wherein the first propagation direction
is opposite of the second propagation direction.
[0007] In another embodiment, the disclosure includes a method
comprising coupling a first polarization component of an incident
optical beam with a TE waveguide mode in a first propagation
direction with a 1D grating element, coupling a second polarization
component of the incident optical beam with a TM waveguide mode in
a second propagation direction with the 1D grating element, wherein
the first propagation direction is opposite of the second
propagation direction, and wherein the first polarization component
is coupled simultaneously with the second polarization
component.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] For a more complete understanding of this disclosure,
reference is now made to the following brief description, taken in
connection with the accompanying drawings and detailed description,
wherein like reference numerals represent like parts.
[0009] FIG. 1 is a schematic cross-section view of a 1D grating
coupler for coupling a single polarization component of an
out-of-plane, optical beam with a waveguide polarization mode.
[0010] FIG. 2 is an elevated view of a 1D grating coupler for
coupling a single polarization component of an out-of-plane,
optical beam with a waveguide polarization mode.
[0011] FIG. 3 is an elevated view of a 2D grating coupler for
simultaneously coupling two polarization components of an
out-of-plane, optical beam with two integrated waveguides.
[0012] FIG. 4A illustrates a side view of an s-polarization
component of an out-of-plane, optical beam incident on an
embodiment of the disclosed grating coupler.
[0013] FIG. 4B illustrates a top view of the s-polarization
component incident on the embodiment of FIG. 4A.
[0014] FIG. 4C illustrates a side view of a p-polarization
component of the out-of-plane, optical beam incident on the
embodiment of FIG. 4A.
[0015] FIG. 4D illustrates a top view of the p-polarization
component incident on the embodiment of FIG. 4A.
[0016] FIG. 5 shows the simulated TE and TM mode transmission
spectra for an embodiment of the disclosed grating coupler with a
650 nanometer (nm) grating period.
[0017] FIG. 6 shows the simulated TE and TM mode transmission
spectra for an embodiment of the disclosed grating coupler with a
520 nm grating period.
DETAILED DESCRIPTION
[0018] It should be understood at the outset that although an
illustrative implementation of one or more embodiments are provided
below, the disclosed systems and/or methods may be implemented
using any number of techniques, whether currently known or in
existence. The disclosure should in no way be limited to the
illustrative implementations, drawings, and techniques illustrated
below, including the exemplary designs and implementations
illustrated and described herein, but may be modified within the
scope of the appended claims along with their full scope of
equivalents.
[0019] Disclosed herein is a system, apparatus, and/or method for
simultaneously coupling a first polarization component of an
optical beam with a TE waveguide mode in a first propagation
direction and a second polarization component of the incident
optical beam with a TM waveguide mode in an opposite second
propagation direction with a 1D grating coupler, wherein the first
propagation direction is opposite of the second propagation
direction. The disclosed 1D grating coupler may provide -2.3 dB
coupling efficiency and a 1 dB bandwidth of greater than 60 nm. The
disclosed 1D grating coupler may be used to combine two
counter-propagating waves comprising orthogonal polarizations to
form an out-of-plane optical beam. Interuniversity Microelectronics
Center (IMEC) fabrication methods and parameters may be used to
fabricate the disclosed 1D grating coupler.
[0020] In an embodiment, the disclosed 1D grating coupler may be
implemented in an optical receiver within a passive optical network
(PON). For example, the PON may be a Next Generation Access (NGA)
system, such as a 10 gigabit per second (Gb/s) GPON (or gigabit
PON) (e.g., XGPON), which may have a downstream bandwidth of about
10 Gb/s and an upstream bandwidth of about 2.5 Gb/s. Alternatively,
the PON may be any Ethernet-based network, such as an EPON (or
Ethernet passive optical network) defined by the Institute of
Electrical and Electronics Engineers (IEEE) 802.3ah standard, a 10
Gb EPON as defined by the IEEE 802.3av standard, an asynchronous
transfer mode PON (APON), a broadband PON (BPON) defined by the
International Telecommunications Union (ITU) Telecommunications
Standardization Sector (ITU-T) G.983 standard, a GPON defined by
the ITU-T G.984 standard, a WDM PON (WPON), or a suitable
after-arising technology, all of which are incorporated by
reference as if reproduced in their entirety.
[0021] FIG. 1 is a schematic cross-section view of a 1D grating
coupler 100 for coupling a single polarization component of an
out-of-plane, optical beam with a waveguide polarization mode.
Grating coupler 100 comprises a substrate 110, an integrated
waveguide 120 formed on the substrate 110, and a 1D grating element
130 coupled to the integrated waveguide 120. Substrate 110 may be a
semiconductor chip, such as a silicon (Si) substrate used in the
fabrication of integrated circuits and microelectronics. Integrated
waveguide 120 may comprise a lower cladding layer 121 and a core
layer 122. Lower cladding layer 121 may consist of an oxide
material (e.g. silicon dioxide) and core layer 122 may consist of a
similar material as substrate 110. Integrated waveguide 120 may be
configured to guide a polarization component of an out-of-plane,
optical beam coupled to a waveguide polarization mode (e.g. TE or
TM) in a propagation direction 126 through total internal
reflection. Total internal reflection may occur due to boundary
conditions imposed on the guided polarization modes by integrated
waveguide 120. The boundary conditions may result from core layer
122 comprising a higher refractive index than both lower cladding
layer 121 and a medium forming an interface opposite of the
interface formed between core layer 122 and lower cladding layer
121 (e.g. an over cladding layer (not shown)). The guided TE
polarization mode may comprise an electric field perpendicular to
the direction of propagation. The guided TM polarization mode may
comprise an electric field parallel to the direction of
propagation. Lower cladding layer 121 may be configured to prevent
a field corresponding to the waveguide polarization mode from
penetrating into substrate 110. 1D grating element 130 may comprise
a plurality of rails 131 and trenches 132 etched into core layer
122. 1D grating element 130 may be configured to couple an
out-of-plane, optical beam 140 with integrated waveguide 120. In an
embodiment, out-of-plane, optical beam 140 may couple with 1D
grating element 130 via free-space propagation and/or an optical
fiber. Out-of-plane, optical beam 140 may comprise two orthogonal
polarization components (e.g. an s-polarization component and a
p-polarization component). Also, the polarization of optical beam
140 in an optical fiber may be randomly polarized, and its
polarization may vary over time.
[0022] 1D grating element 130 may only be able to efficiently
couple one of the polarization components to a waveguide
polarization mode of integrated waveguide 120. Also, the electric
field direction of the TE polarization mode of integrated waveguide
120 may naturally align with the electric field of the
s-polarization component. Thus, 1D grating coupler 100 may be
considered a polarization sensitive device, and the s-polarization
component of the out-of-plane, optical beam 140 may be coupled to
the TE polarization mode of integrated waveguide 120. However, the
TM polarization mode of integrated waveguide 120 may have an
electric field direction that may naturally align with the
p-polarization component in a counter-propagating direction.
[0023] FIG. 2 is an elevated view of a 1D grating coupler 200 for
coupling a single polarization component of an out-of-plane,
optical beam 240 with a waveguide polarization mode. In an
embodiment, an out-of-plane, optical beam 240, comprising an
s-polarization component 242 and a p-polarization component 244,
may be directed towards a 1D grating coupler 200. Out-of-plane,
optical beam 240 may be directed at an incident angle 246 with
respect to a line 248 normal to the grating coupler surface plane
(e.g. 10 degrees (.degree.)). 1D grating coupler 200 can comprise a
1D grating element 230, which may be similar to grating element 130
of FIG. 1. 1D grating element 230 of grating coupler 200 can be
used to separate the s-polarization component 242 of the optical
beam 240 from the p-polarization component 244. 1D grating element
230 may also couple the s-polarization component 242 into a TE
polarization mode of an integrated waveguide 220, which may be
similar to grating coupler 100 of FIG. 1, in a propagation
direction 226. Integrated waveguide 220 can guide the s-propagation
component 242 of the out-of-plane, optical beam 240 along the
propagation direction 226 for further processing.
[0024] FIG. 3 is an elevated view of a 2D grating coupler 300 for
simultaneously coupling two polarization components of an
out-of-plane, optical beam 340 with two integrated waveguides (e.g.
first integrated waveguide 320A and second integrated waveguide
320B). In an embodiment, an out-of-plane, optical beam 340,
comprising an s-polarization component 342 and a p-polarization
component 344, may be directed towards a 2D grating coupler 300.
Out-of-plane, optical beam 340 may be directed at an incident angle
346 with respect to a line 348 normal to the grating coupler
surface (e.g. 10.degree.). 2D grating coupler 300 may comprise a 2D
grating element 330, comprising an array of holes 332 (or
alternatively an array of posts (not shown)). 2D grating element
330 can be formed at an intersection of a pair of orthogonal
integrated waveguides (e.g. first integrated waveguide 320A and
second integrated waveguide 320B). 2D grating element 330 may be
configured to separate the s-polarization component 342 of the
optical beam 340 from the p-polarization component 344. 2D grating
element 330 may further be configured to couple the s-polarization
component 342 into a TE polarization mode of the first integrated
waveguide 320A in a first propagation direction 326A. Also, 2D
grating element 330 may couple the p-polarization component 344 of
the optical beam 340 into a TE polarization mode of the second
integrated waveguide 320B in a second propagation direction 326B. A
person skilled in the art may refer to this process of separating
polarization components of an optical beam into a plurality of
waveguides as polarization splitting. First integrated waveguide
320A and second integrated waveguide 320B may guide their
respective polarization components of the out-of-plane, optical
beam 340 along their respective propagation directions to be
separately processed further before they are sent to a detector. 2D
grating couplers (e.g. 2D grating coupler 300) may experience lower
coupling efficiencies of less than 3.5 dB and reduced 1 dB
bandwidths (e.g. less than 40 nm) than 1D grating couplers (e.g. 1D
grating coupler 200).
[0025] FIGS. 4A-4D illustrate an embodiment of a 1D grating coupler
400 for simultaneously coupling two polarization components of an
out-of-plane, optical beam incident with separate integrated
waveguides. FIG. 4A illustrates a side view of an s-polarization
component 442 of the out-of-plane, optical beam incident on an
embodiment of a 1D grating coupler 400. FIG. 4B illustrates a top
view of the s-polarization component 442 incident on the embodiment
of FIG. 4A. FIG. 4C illustrates a side view of a p-polarization
component 444 of the out-of-plane, optical beam incident on the
embodiment of FIG. 4A. FIG. 4D illustrates a top view of the
p-polarization component 444 incident on the embodiment of FIG. 4A.
1D grating coupler 400 may be fabricated using similar process
parameters and similar layer thickness as employed by IMEC. In an
embodiment, 1D grating coupler 400 may be fabricated through such
techniques as liquid-phase epitaxy (LPE), vapor-phase epitaxy
(VPE), and/or molecular-beam epitaxy (MBE).
[0026] In FIGS. 4A-4D, 1D grating coupler 400 may comprise a
substrate 410, an integrated waveguide 420 formed on the substrate
410, and a grating element 430 coupled to integrated waveguide 420.
Substrate 410 may be substantially similar to substrate 110 of FIG.
1. As shown in FIGS. 4B and 4D, integrated waveguide 420 may
comprise, a first integrated waveguide 420A in a first direction
451 from line 450 and a second integrated waveguide 420B in a
second direction 452 from line 450, which may be opposite of first
direction 451. First integrated waveguide 420A and second
integrated waveguide 420B may both comprise a lower cladding layer
421 and a core layer 422, which may be similar to lower cladding
layer 121 and core layer 122. Grating element 430 may be configured
to separate the s-polarization component 442 of an optical beam
from the p-polarization component 444. Grating element 430 may be
further configured to simultaneously couple the s-polarization
component 442 into a TE polarization mode of first integrated
waveguide 420A propagating in the first direction 451 and the
p-polarization component 444 into a TM polarization mode of second
integrated waveguide 420B propagating in the second direction 452.
As shown in FIG. 4A, s-polarization component 442 may comprise an
electric field direction that naturally matches the TE polarization
mode electric field shown in FIG. 4B. Also, as shown in FIG. 4C,
p-polarization component 444 may comprise an electric field
direction that naturally matches the TM polarization mode electric
field shown in FIG. 4D. Efficient coupling of a specific
polarization component with a corresponding waveguide polarization
mode, may be realized when:
Equation 1 n c cos .theta. = n eff - .lamda. .LAMBDA. ( 1 )
##EQU00001##
[0027] Where: [0028] .eta..sub.c=an upper cladding refractive index
[0029] .theta.=an incident light tilt angle [0030] .eta..sub.eff=a
waveguide effective index [0031] .lamda.=an incident light
wavelength [0032] .LAMBDA.=a grating period
[0033] Furthermore, a diffraction angle corresponding to the TE
polarization mode of an integrated waveguide (.theta..sub.TE) may
not be the same as a diffraction angle corresponding to the TM
polarization mode of the integrated waveguide (.theta..sub.TM).
This difference between .theta..sub.TE and .theta..sub.TM may
correspond to different effective indices for the TE and TM
polarization modes (i.e. .eta..sub.eff.sup.TE and
.eta..sub.EFF.sup.TM, respectively). Simultaneously coupling the
s-polarization and p-polarization components of the out-of-plane
optical beam to counter-propagating TE and TM polarization modes of
integrated waveguide 420 may be achieved when:
Equations 2 and 3
.theta..sub.TE+.theta..sub.TM=180.degree. (2)
[0034] Where: [0035] .theta..sub.TE=a diffraction angle for a TE
waveguide polarization mode of an out-of-plane optical beam [0036]
.theta..sub.TM =a diffraction angle for a TM waveguide polarization
mode of the out-of-plane optical beam
[0036] .phi. = ( .theta. TE + .theta. TM ) 2 ( 3 ) ##EQU00002##
[0037] Where: [0038] .PHI.=an incident angle of the out-of-plane
optical beam with respect to a line normal to the grating coupler
surface
[0039] This relationship may be realized when grating element 430
comprises a grating period 434 defined by:
Equation 4 .LAMBDA. = 2 .lamda. n eff TE + n eff TM ( 4 )
##EQU00003##
[0040] Where: [0041] .eta..sub.eff.sup.TE=a TE polarization
waveguide effective index [0042] .eta..sub.eff.sup.TM=a TM
polarization waveguide effective index
[0043] As shown by equations 3 and 4, grating period 434 may be
independent of the out-of-plane, optical beam incident angle. Also,
grating period 434 may be designed according to the wavelength of
the out-of-plane, optical beam being coupled.
[0044] Process variations of one or more structural parameters of
grating element 430 may result in both .eta..sub.eff.sup.TE and
.eta..sub.eff.sup.TM shifting in tandem. For example, fabrication
imperfections varying a layer thickness of grating element 430, a
width of rails 431, or an etch depth of trenches 432 may result in
shifts for both .eta..sub.eff.sup.TE and .eta..sub.eff.sup.TM.
Grating period 434 may still simultaneously couple both
polarization components of an out-of-plane, optical beam at a
different wavelength to counter-propagating waveguide polarization
modes notwithstanding such variations. In an embodiment, grating
element 430 may be apodized by varying an occupation ratio and/or a
spatial period in each section of the grating element to optimize
the shape of a free-space optical beam. In some embodiments,
varying the incident angle of the out-of-plane, optical beam may
not compensate for a wavelength shift resulting from process
variations. The transmission spectra of the TE and TM waveguide
polarization modes may shift in opposite directions in these
embodiments.
[0045] Simultaneously coupling two orthogonal polarization
components of an out-of-plane, optical beam may reduce the
polarization sensitivity of the disclosed 1D grating coupler 400.
As a result, 1D grating coupler 400 may be used for polarization
diversity coupling and/or polarization multiplexing. In an
embodiment, polarization multiplexing may be used to combine two
groups of optical channels, which may double the capacity of an
optical fiber without introducing wavelength sensitive
elements.
[0046] FIGS. 5 and 6 show the transmission spectrum results from a
three-dimensional (3D) finite-difference time-domain (FDTD)
simulation of an embodiment of the disclosed grating coupler, e.g.,
1D grating coupler 400 of FIGS. 4A-4D.
[0047] FIG. 5 shows the simulated TE and TM waveguide polarization
mode transmission spectra for an embodiment of the disclosed
grating coupler with a 650 nm grating period. In FIG. 5, the y-axis
represents the percentage of an optical beam polarization component
coupled to a corresponding waveguide polarization mode and the
x-axis represents a wavelength measured in micrometres (.mu.m). The
grating coupler embodiment of FIG. 5 comprises a grating height of
380 nm, an etch depth of 220 nm, an occupation ratio for the
silicon rails of 36%. A waveguide comprising a height of 220 nm and
a width of 10 .mu.m, was coupled to the grating coupler. A 1550 nm
Gaussian optical beam with a waist radius of 6 .mu.m and an
incident angle of 13.4.degree. with respect to a line normal to the
grating coupler surface was used for the simulation. As shown, a
peak maxima centered at approximately 1550 nm of a TE waveguide
polarization mode provides an approximate -2.1 dB coupling
efficiency with a 1 dB bandwidth of 62.5 nm. Also, a peak maxima
centered around 1550 nm of a TM waveguide polarization mode
provides an approximate -2.3 dB coupling efficiency with a 1 dB
bandwidth of 73 nm. FIG. 5 also shows that the disclosed grating
coupler provides a 3 dB bandwidth that fully encompasses a commonly
used wavelength range for photonics work (C-band), which spans from
1530 nm to 1565 nm and may be used for long-range optical
communications.
[0048] FIG. 6 shows the simulated TE and TM waveguide polarization
mode transmission spectra for an embodiment of the disclosed
grating coupler with a 520 nm grating period. In FIG. 6, the y-axis
represents the percentage of an optical beam polarization component
coupled to a corresponding waveguide polarization mode and the
x-axis represents a wavelength measured in nm. The grating coupler
embodiment of FIG. 6 comprises a grating height of 380 nm, an etch
depth of 220 nm, an occupation ratio for the silicon rails of 35%.
A waveguide, comprising a height of 220 nm and a width of 9 .mu.m,
was coupled to the grating coupler. A Gaussian optical beam with a
waist radius of 6 .mu.m and an incident angle of 13.4.degree. with
respect to a line normal to the grating coupler surface was used
for the simulation. As shown, a peak maxima centered around 1290 nm
for the TE waveguide polarization mode provides an approximate
-2.15 dB coupling efficiency with a 1 dB bandwidth of 46 nm. Also,
a peak coupling efficiency of the TM waveguide polarization mode is
approximately -2.3 dB with a 1 dB bandwidth of 56.5 nm. FIG. 6 also
shows that the disclosed grating coupler provides a 3 dB bandwidth
of approximately 42 nm around 1300 nm, which may be used for
short-range optical communications.
[0049] At least one embodiment is disclosed and variations,
combinations, and/or modifications of the embodiment(s) and/or
features of the embodiment(s) made by a person having ordinary
skill in the art are within the scope of the disclosure.
Alternative embodiments that result from combining, integrating,
and/or omitting features of the embodiment(s) are also within the
scope of the disclosure. Where numerical ranges or limitations are
expressly stated, such express ranges or limitations should be
understood to include iterative ranges or limitations of like
magnitude falling within the expressly stated ranges or limitations
(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater
than 0.10 includes 0.11, 0.12, 0.6, etc.). For example, whenever a
numerical range with a lower limit, Rl, and an upper limit, Ru, is
disclosed, any number falling within the range is specifically
disclosed. In particular, the following numbers within the range
are specifically disclosed: R=Rl+k*(Ru-Rl), wherein k is a variable
ranging from 1 percent to 100 percent with a 1 percent increment,
i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, .
. . , 50 percent, 51 percent, 52 percent, . . . , 95 percent, 96
percent, 97 percent, 98 percent, 99 percent, or 100 percent.
Moreover, any numerical range defined by two R numbers as defined
in the above is also specifically disclosed. Use of the term
"optionally" with respect to any element of a claim means that the
element is required, or alternatively, the element is not required,
both alternatives being within the scope of the claim. Use of
broader terms such as comprises, includes, and having should be
understood to provide support for narrower terms such as consisting
of, consisting essentially of, and comprised substantially of.
Accordingly, the scope of protection is not limited by the
description set out above but is defined by the claims that follow,
that scope including all equivalents of the subject matter of the
claims. Each and every claim is incorporated as further disclosure
into the specification and the claims are embodiment(s) of the
present disclosure. The discussion of a reference in the disclosure
is not an admission that it is prior art, especially any reference
that has a publication date after the priority date of this
application. The disclosure of all patents, patent applications,
and publications cited in the disclosure are hereby incorporated by
reference, to the extent that they provide exemplary, procedural,
or other details supplementary to the disclosure.
[0050] While several embodiments have been provided in the present
disclosure, it should be understood that the disclosed systems and
methods might be embodied in many other specific forms without
departing from the spirit or scope of the present disclosure. The
present examples are to be considered as illustrative and not
restrictive, and the intention is not to be limited to the details
given herein. For example, the various elements or components may
be combined or integrated in another system or certain features may
be omitted, or not implemented.
[0051] In addition, techniques, systems, subsystems, and methods
described and illustrated in the various embodiments as discrete or
separate may be combined or integrated with other systems, modules,
techniques, or methods without departing from the scope of the
present disclosure. Other items shown or discussed as coupled or
directly coupled or communicating with each other may be indirectly
coupled or communicating through some interface, device, or
intermediate component whether electrically, mechanically, or
otherwise. Other examples of changes, substitutions, and
alterations are ascertainable by one skilled in the art and could
be made without departing from the spirit and scope disclosed
herein.
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